1. Field of the Invention
The present invention relates to a coated material having an isotropic coating layer that is obtained by peeling a portion of a film press-fitted to a base material such as a metal foil and made of a polymer capable of forming an optically anisotropic melt phase, and a method of making the coated material. The present invention also relates to a metal-polymer laminate obtained by peeling a portion of the polymer film firmly sandwiched between metal foils, in a direction thicknesswise thereof, and a method of making the metal-polymer laminate.
In the description of the present invention, the polymer capable of forming an optically anisotropic melt phase is referred to as a xe2x80x9cliquid crystal polymerxe2x80x9d; the film made of the liquid crystal polymer is referred to as a xe2x80x9cliquid crystal polymer filmxe2x80x9d; and the coated material is intended to mean an article of manufacture formed with the liquid crystal polymer coating layer by coating the liquid crystal polymer to the base material.
2. Description of the Prior Art
The liquid crystal polymer has been well known which has various advantageous features including (1) a capability of being thermally bonded directly to a metallic foil layer; (2) a high resistance to heat; (3) a low moisture absorbability; (3) an excellent dimensional stability to thermal change in size; (5) an excellent resistance to change in size brought about by moisture; (6) an excellent property in high frequency characteristic; (7) a fire-proof property with no need to add a flame retardant containing a toxic halogen, phosphorus, antimony and others; (8) an excellent resistance to chemicals; (9) an excellent resistance to radiations; (10) having a controllable thermal expansion coefficient; (11) a flexibility at low temperatures; (12) a property of a high gas barrier (a considerably low permeability to a gaseous material such as, for example, oxygen), and so on.
In recent years, demands have been arisen to use such excellent liquid crystal polymer as a coating material to be applied in the form of a thin film to a metallic foil layer, a silicon plate or a ceramics plate to provide a base material for a precision circuit substrate, a multi-layered circuit substrate, a sealing material or a package can. In addition, because of the resistance to heat and chemicals, the low moisture absorbability and the gas barrier property, demands have increased for the use of the liquid crystal polymer as a coating material that can be utilized to form a protective layer on a metal susceptible to corrosion.
The first problem in utilizing the liquid crystal polymer as a coating material will first be discussed:
To form a thin skin film of, for example, a synthetic resin over the surface of an article of manufacture, various methods have been known such as, for example, a lining process and a coating process. The lining process and the coating process are known to be distinct from each other because of the following reasons. Specifically, the coating process has a primary objectivity in decorative purpose to form a continuous skin film over the article to thereby protect the article from corrosion and contamination and also to provide the article with an appealing ornament and is also often used to form a film for imparting a non-adhesive property and a low frictional property. On the other hand, the lining process is a process of forming a protective thick film on vessels (baths) and tubes or pipes that are used in the chemically and/or physically severe environment where corrosion and/or erosion are strictly desired to be avoided. However, the coating and lining processes have features so common to each other that the line of distinction can hardly be drawn therebetween. It is generally recognized that the skin layer having a thickness of 0.5 mm or more is classified as a lining whereas the skin film having a thickness of 0.5 mm or less is classified as a coating. It is also generally recognized that the coating is to form a film of several tens microns mainly on a surface of a structure whereas the lining is to form a film of several hundreds microns.
In either case, the present invention pertains to a technique of forming on a base material a very thin film of a liquid crystal polymer to a thickness of 25 xcexcm or less, particularly 15 xcexcm or less and may therefore be said to pertain to the coating technique.
As one of important properties of the coating, attention is centered on the durability of the coating against change in temperature and, however, this leads to a problem of how the difference in thermal expansion coefficient between the coating and the base material bearing the coating should be resolved. While the coating process includes, inter alia, (1) a dipping method, (2) a flow coating method, (3) a curtain coating method, (4) a roll coating method, (5) an electro-deposition method, (6) a brush coating method, (7) a spray coating method and (8) a gas-phase coating method, none of these known methods can be utilized to form a film of a liquid crystal polymer by the following reason. Specifically, due to a unique property of the liquid crystal polymer in that molecules of the liquid crystal polymer have a propensity of orienting in the same direction, the liquid crystal polymer molecules tend to be oriented in the same direction when a force is applied to a molten liquid crystal polymer being applied to form a thin film. Considering that the physical property such as thermal expansion coefficient measured in a direction conforming to the molecular orientation is considerably different from that measured in a direction transverse to the molecular orientation, that is, the liquid crystal polymer has an anisotropy, it is impossible to render the coating-bearing base material and the layer of the liquid crystal polymer to have the same or substantially same thermal expansion coefficient in all directions on a plane. Although the prior art coating process is capable of providing a liquid crystal polymer layer of a large thickness of, for example, 50 xcexcm or more, it has been found that the resultant liquid crystal polymer layer has an anisotropy and is therefore incapable of being used as a practically utilizable coating. Nonetheless, no technique of forming a layer of a liquid crystal polymer having an improved isotropy to a thickness of 15 xcexcm or less have hitherto been made available.
The second problem will now be discussed:
Circuit substrates or the like that are generally utilized in the field of electronics make use of a metal-resin laminate prepared by press-bonding together a foil layer of an electroconductive metal and a film-like insulating material (a film or a sheet with or without a metallic foil layer coated thereon) having an electrically insulating property. The metal-resin laminate is available in the form of a double-sided metal-resin laminate in which an electric insulating layer is sandwiched between two metallic foil layers and of a single-sided met al-resin laminate in which a single metallic foil layer and a single electric insulating layer are bonded together. The liquid crystal polymer having the excellent properties as discussed above is generally recognized as an ideal material for the electric insulating layer used in the laminates.
As a method of making a metal-polymer laminate without diminishing the excellent properties of the liquid crystal polymer, various methods have been well known. (i) Specifically, in the case of the double-sided metal-polymer laminate, the method is known to comprise sandwiching a liquid crystal polymer layer between two metallic foil layers, and hot-pressing the resultant sandwich structure with the use of a hot plate or a hot roll to cause the liquid crystal polymer to melt so that the metallic foil layers and the liquid crystal polymer layer can be thermally bonded together to eventually provide the double-sided metal-polymer laminate upon solidification of the liquid crystal polymer. (ii) On the other hand, in the case of the single-sided metal-polymer laminate, the method is known to comprise sandwiching a liquid crystal polymer layer between a single metallic foil layer and a single release film, hot-pressing the resultant sandwich structure with the use of a hot plate or a hot roll to cause the liquid crystal polymer to melt so that the metallic foil layer and the liquid crystal polymer layer can be thermally bonded together, and removing the release film to leave the resultant single-sided metal-polymer laminate.
While the prior art method of making the double-sided metal-polymer laminate is satisfactory, the prior art method of making the single-sided metal-polymer laminate has a problem in that since the release film has to be removed and is then discarded, manufacture of the single-sided metal-polymer laminate tends to be costly. Moreover, since a high temperature of about 300xc2x0 C. is required to melt the liquid crystal polymer in the practice of making the single-sided metal-polymer laminate, the release film used must be chosen from highly heat-resistant films made of an expensive resinous material such as, for example, Teflon(copyright) or polyimide, and increase of the manufacturing cost brought about by the use of the expensive release film has made it very difficult to manufacture the single-sided metal-polymer laminate on a commercially profitable basis.
On the other hand, demand has arisen particularly from electronics concerns for availability of circuit substrates having a more reduced thickness. Since as discussed hereinabove the liquid crystal polymer is suited as a material for the electric insulating layer, realization of a circuit substrate comprising a thin layer of the liquid crystal polymer and a thin foil layer of metal is persistently longed for.
Although a film of the liquid crystal polymer is often required for the thin liquid crystal polymer layer, the liquid crystal polymer film if manufactured in a usual manner would have a molecular orientation dominantly in one direction. The film having the molecular orientation dominantly in one direction tends to be easily torn in a direction parallel to the molecular orientation and also tends to exhibit considerably differing thermal changes in shape in respective directions parallel to and transverse to the molecular orientation, that is, to exhibit an anisotropic film. Therefore the liquid crystal polymer hitherto available in the market can hardly be used as a material for the electric insulating layer in the circuit substrate. However, as discussed in connection with the first problem, it is difficult to form a film having a thickness of 15 xcexcm or less with an isotropic liquid crystal polymer for use as a material for the electric insulating layer, and much difficulty has hitherto been encountered to form a film having a thickness of 10 xcexcm or less. No report has hitherto been made yet which show a success in making the isotropic liquid crystal polymer film having a film thickness not greater than 10 xcexcm.
Accordingly, the present invention has been devised with a view to substantially eliminating the above discussed first problem inherent in the prior art coating methods and is intended to provide a means for forming the liquid crystal polymer coating, particularly the liquid crystal polymer coating having a reduced thickness, in which a problem associated with the anisotropic molecular orientation is resolved, that is, having an improved isotropy to thereby substantially eliminate the first problem discussed hereinbefore.
Another important object of the present invention is to provide an improved method of making the single-sided metal-polymer laminate in which no release film is required, to thereby substantially eliminate the second problem discussed hereinbefore.
A further important object of the present invention is to provide an improved laminate comprising a metallic foil layer and an electric insulating layer prepared from the liquid crystal polymer from which a problem associated with the anisotropic molecular orientation is resolved.
According to the coating method of the present invention, a film prepared from a liquid crystal polymer and having a segment orientation ratio (SOR) of not greater than 1.3 is hot-pressed and, hence, bonded to a base material to provide a layered structure. The liquid crystal polymer film is subsequently peeled off from the layered structure so as to leave a thin ply of the liquid crystal polymer on the base material. In this way, a thin liquid crystal polymer coating can easily be obtained.
A coated material having a thin coating of the liquid crystal polymer left on the base material according to the present invention comprises a coating of the liquid crystal polymer capable of forming a melt layer having an optically anisotropy, in which the segment orientation ratio of the liquid crystal polymer coating is not greater than 1.3. In a preferred embodiment of the coated material of the present invention, the coating layer referred to above has a thickness not greater than 15 xcexcm. Accordingly, the coating in the coated material of the present invention has an isotropic molecular orientation while securing a small thickness, and therefore, the coated material of the present invention can advantageously be used as a material for the precision circuit substrates, the multi-layered circuit substrates, sealing members, package cans with maximized utilization of the excellent properties of the liquid crystal polymer discussed hereinbefore.
The term xe2x80x9csegment orientation ratioxe2x80x9d used hereinbefore and hereinafter is an index descriptive of the degree of orientation of molecules forming a segment and represents, unlike the standard MOR (molecular orientation ratio), a value in which the thickness of an object is taken into consideration. This segment orientation ratio can be calculated in the following manner.
Using a commercially available microwave molecular orientation degree measuring apparatus shown in FIG. 6 and available from the KS Systems Corporation, the intensity (the microwave penetration strength) of an electric field of the microwaves transmitted through a liquid crystal polymer film is first measured.
The measuring apparatus indicated by 61 in FIG. 6 comprises a microwave generator 63 capable of generating microwaves MW of a predetermined wavelength that irradiate a liquid crystal polymer film 65, a microwave resonant waveguide 64 and a penetration strength detecting means 68. The microwave resonant waveguide 64 has an intermediate portion thereof at which the liquid crystal polymer film 65 is set so as to lie vertically with respect to the direction of propagation of the microwaves MW. The liquid crystal polymer film 65 so placed is retained for rotation in the direction R in a plane perpendicular to the direction of propagation of the microwaves MW by means of a rotary drive mechanism (not shown). The microwave resonant waveguide 64 includes a pair of reflecting mirrors 67 disposed at respective opposite ends of the waveguide 64 for reflecting the microwaves MW penetrating through the liquid crystal polymer film 65 to allow the waveguide 64 to undergo resonance. The strength of the microwaves which have penetrated through the liquid crystal polymer film 65 can be detected by the penetration strength detecting means 68. This penetration strength detecting means 68 includes a detecting element 68a disposed at a predetermined position rearwardly within the microwave resonant waveguide 64 to measure the microwave penetration strength.
Based on the resultant measurement, an m value (hereinafter referred to as an xe2x80x9cindex of refractionxe2x80x9d) can be calculated from the following equation:
m=(Z0/xcex94z)xc3x97(1xe2x88x92xcexdmax/xcexd0)
wherein Z0 represents a device constant, xcex94z represents an average thickness of an object subjected to the measurement, xcexdmax represents the frequency at which the maximum microwave penetration strength can be obtained when the frequency of the microwaves is changed, and xcexd0 represents the frequency at which the maximum microwave penetration strength can be obtained when the average thickness is zero, that is, when no object is present.
The segment orientation ratio (SOR) can be calculated from the following equation:
SOR=m0/m90
wherein m0 represents a value of the m value which is exhibited when the angle of rotation of the object in a direction indicated by R in FIG. 6 relative to the direction of oscillation of the microwaves is 0xc2x0, that is, when the direction of oscillation of the microwaves is aligned with the direction in which molecules of the object are most oriented and in which the minimum microwave penetration strength is exhibited, and m90 represents a value of the m value which is exhibited when the angle of rotation of the object in the direction R is 90xc2x0.
The SOR of ideal isotropic films exhibits 1, while the SOR of a liquid crystal polymer film prepared by the use of the standard T-die film forming method and in which molecules are oriented strongly in one direction exhibits about 1.5. The SOR of the isotropic film obtained by the use of the standard isotropic inflation film forming method is not greater than 1.3.
The liquid crystal polymer includes all kinds of liquid crystal polymer such as Half-1 liquid crystal polymer, Whole-1 liquid crystal polymer, Half-2 liquid crystal polymer, and Whole-2 liquid crystal polymer. See xe2x80x9cSeikeixc2x7Sekkei no tameno Ekisho Porimah (Liquid Crystal Polymer for Molding and Design)xe2x80x9d authored by Junichi Suenaga and available from the Sigma Publishing Co.
In any event, examples of the liquid crystal polymer include, for example, well known thermotropic liquid crystal polyester and thermotropic liquid crystal polyester amide prepared from compounds, such as classified under (1) to (4) below, and their derivatives. It is, however, pointed out that to prepare a liquid crystal polymer, various raw material compounds have their proper combination and amount carefully chosen.
(1) Aromatic or aliphatic dihydroxy compounds, representative examples of which are shown in Table 1 below.
(2) Aromatic or aliphatic dicarboxylic acids, representative examples of which are shown in Table 2 below.
(3) Aromatic hydroxycarboxylic acid, representative examples of which are show in Table 3 below.
(4) Aromatic diamines, aromatic hydroxyamines and aromatic aminocarboxylic acids, representative examples of which are shown in Table 4 below.
(5) Representative examples of the liquid crystal polymers prepared from any of those starting material compounds include copolymers having such structural units as indicated by (a) to (e) in Table 5 below.
Those liquid crystal polymers preferably have a transition temperature to an optically anisotropic melt phase within the range of from about 200 to about 400xc2x0 C., more preferably from about 250 to about 350xc2x0 C., so that the resultant film can have a desirable heat resistance and a desirable processability. Unless physical properties of the liquid crystal polymer film are impaired, one or a mixture of various additives such as a smoothing agent, an antioxidant and a filler may be added thereto if desired.
The film prepared from any of those liquid crystal polymers discussed above can be made by the use of one or a combination of the well-known T-die process and the well-known inflation process. Particularly with the inflation method, stresses can be applied not only in a direction of the mechanical axis of the film (which direction is hereinafter referred to as the MD direction), but also in a direction (hereinafter referred to as TD direction) perpendicular to the MD direction and, therefore, the inflation method is effective to eventually manufacture the liquid crystal polymer film having balanced physical and thermal properties in both of the MD and TD directions.
Accordingly, one of important features of the present invention lies in the use of the isotropic liquid crystal polymer film as a coating material, and even if the anisotropic liquid crystal polymer film having the segment orientation ratio (SOR) greater than 1.3 is used as a coating material and even if after this coating material has been applied, the anisotropic liquid crystal polymer coating layer is heated to melt, this will not transform into an isotropic liquid crystal polymer coating layer. This is a fundamental behavior of molecules of the liquid crystal polymer, and the inventors of the present invention have experimentally confirmed that even though the anisotropic liquid crystal polymer coating layer is heated to a temperature 35xc2x0 C. higher than the melting point of the liquid crystal polymer, it would not transform into the isotropic liquid crystal polymer.
The base material to which the liquid crystal polymer film is applied may be made of any inorganic material such as, for example, metal, glass or ceramics, or any organic material such as, for example, plastics, wood, textile fibers, provided that those materials have a softening point higher than the temperature at which the liquid crystal polymer is fusion-bonded thereto. It is to be noted that the liquid crystal polymer itself can be included as a material for the base material. By way of example, for the purpose of improving surface characteristics (such as, for example, bondability, physical properties, frictional resistance, surface wettability, gas barrier property, resistance to chemicals, resistance to solvents, affinity to solvents, aesthetic appearance and so on) of the base material mixed with a reinforcement such as filler or glass cloth or without filler or glass cloth, the liquid crystal polymer coating layer may be provided on the surface of the base material.
In particular, the liquid crystal polymer coating layer of the present invention is suited for retention of electronic components or electronic circuits forming an electronic board and, in such cases, the metallic foil layer may often comprise the base material. Material for the metallic foil may be selected from metals of a kind used in electric connections and is preferably chosen from the group consisting of, for example, gold, silver, copper, nickel, aluminum, iron, steel, tin, brass, magnesium, molybdenum, a copper-nickel alloy, a copper-beryllium alloy, a nickel-chromium alloy, a silicon carbide alloy, graphite and a mixture thereof.
Another one of the important feature of the present invention lies in that after a film of the isotropic liquid crystal polymer has been hot-pressed to the base material, the film is peeled off from the base material so as to leave a thin film of the liquid crystal polymer on the base material. While this is difficult with ordinary polymers, this type of coating can be accomplished only when the intra-layer separability which is a unique property of the liquid crystal polymer film is utilized. The term xe2x80x9cintra-layer separabilityxe2x80x9d means a capability of the liquid crystal polymer film being internally separated to produce mica-like thin flakes of liquid crystal polymer. In order for the isotropic liquid crystal polymer film to be hot-pressed and, hence, fusion-bonded to the base material while making maximized use of the excellent intra-layer separability, the heating temperature should not be increased to a value equal to or higher than the melting point of the liquid crystal polymer.
The liquid crystal polymer coating layer so formed on the surface of the base material by the method described above may, when subsequently heated to the temperature not lower than the melting point, loose the intra-layer separability. Also, where after the liquid crystal polymer coating layer and any other material have been overlapped together in face-to-face relation with each other the base material and such any other material are hot-pressed together at a temperature not lower than the melting point of the liquid crystal polymer, no intra-layer separation occur in the liquid crystal polymer coating layer since the liquid crystal polymer coating layer is heated to a temperature not lower than the melting point thereof during a hot-pressing process.
The coated material having the isotropic liquid crystal polymer according to the present invention has the liquid crystal polymer coating layer which preferably has a thickness not greater than 15 xcexcm.
The film forming method for forming the liquid crystal polymer film is one of the high-tech methods and manufacture of the thin film is difficult to accomplish without incurring an increase in manufacturing cost. In general, manufacture of the liquid crystal polymer coating layer of 20 xcexcm or greater is relatively easy and, depending on the circumstances, the liquid crystal polymer coating layer can be obtained by a simple manufacture which does not require a peeling process to induce the intra-layer separation. Nevertheless, it is often considered important to prepare the liquid crystal polymer coating layer of 20 xcexcm or greater in thickness through the peel-off process since when the thick liquid crystal polymer coating layer is internally separated by peeling off, the coating layer will have a rough surface effective to retain a bonding material.
However, the coated material according to the present invention is effectively utilized where in its application to the electronic circuit substrates and their related component parts a thin liquid crystal polymer coating layer is required. Accordingly, in order to provide the isotropic liquid crystal polymer coating layer particularly having a thickness not greater than 20 xcexcm, preferably not greater than 15 xcexcm, the coating method herein disclosed and claimed in accordance with the present invention is the sole effective method to accomplish it. With the coating method of the present invention, the liquid crystal polymer coating layer may have an extremely small average thickness, the minimum value of which is close to zero, and the liquid crystal polymer coating layer having an average thickness of, for example, 1 xcexcm or smaller can easily be manufactured. Under precisely controlled conditions, it is possible to manufacture the isotropic liquid crystal polymer coating layer having an average thickness of 0.1 xcexcm or smaller.
In the coated material having the isotropic liquid crystal polymer coating layer according to the present invention, the isotropic liquid crystal polymer coating layer and the base material on which the isotropic liquid crystal polymer coating layer is applied preferably have the same or substantially same thermal expansion coefficient.
As discussed in connection with the problems inherent in the prior art, the thermal expansion coefficient of the liquid crystal polymer coating layer is preferably as close to that of the base material as possible. In particular, if the dimensional change between the base material and the coating layer relative to a temperature change of 100xc2x0 C. is 0.2% or less, the liquid crystal polymer coating layer can satisfactorily be used as a precise coating material for the electronics component parts. Accordingly, the fact that the thermal expansion coefficient of the isotropic liquid crystal polymer coating layer and that of the base material are substantially equal to each other speaks of xc2x120 ppm/xc2x0 C. (that is, xc2x1(2/1,000)%/xc2x0 C.). Thus, the simplest method to render the thermal expansion coefficient of the liquid crystal polymer coating layer to be as close to that of the base material as possible is to make the thermal expansion coefficient of the liquid crystal polymer film, which serves as a material for the coating layer, equal to that of the base material. However, even though the liquid crystal polymer film and the base material have respective thermal expansion coefficients different from each other, heat treatment of the liquid crystal polymer coating layer formed by the use of the liquid crystal polymer film is effective to render the respective thermal expansion coefficients to be substantially equal to each other. If during the heat treatment the heating temperature is very precisely controlled, it is possible to allow the base material and the coating layer to have the respective thermal expansion coefficients which substantially match with each other within a measurement tolerance. In order to control the thermal expansion coefficient where the standard thermoplastic polymer having no liquid crystal phase or the standard thermosetting resin such as epoxy resin is employed for the coating layer, special operations are required to add an inorganic powder or an inorganic cloth to the coating layer, and to control the proportion of the additive in the coating layer or to control the cross-linking density of polymer molecules forming the coating layer. However, in the case of the liquid crystal polymer coating layer, it can easily be accomplished with the simple heat treatment owing to the unique properties of the liquid crystal polymer molecules.
As described above, the present invention makes use of the unique feature of the liquid crystal polymer in which the molecules of the liquid crystal polymer can easily be oriented and can exhibit the excellent intra-layer separability when formed into a film. Because of this, the thin liquid crystal polymer coating layer of the present invention can be easily formed by initiating the intra-layer separation when the liquid crystal polymer film hot-pressed to the surface of the base material is being peeled off from the base material, so as to leave a fraction of the thickness of the liquid crystal polymer film on the base material.
The method of making the metal-polymer laminate of the present invention which will be described subsequently is closely associated with the liquid crystal polymer coating method of the present invention in that the intra-layer separability of the liquid crystal polymer is utilized, provided that the base material in the above described liquid crystal polymer coating comprises a metallic foil.
The method of making the single-sided metal-polymer laminate according to the present invention can be practiced by separating the double-sided metal-polymer laminate, which comprises the layer of the liquid crystal polymer having its opposite surfaces bonded with upper and lower metallic foils, into first and second single-sided metal-polymer laminates. The resultant first single-sided metal-polymer laminate comprises the upper metal foil having a lower surface to which a portion of the liquid crystal polymer layer is bonded whereas the resultant second single-sided metal-polymer laminate comprises the lower metal foil having an upper surface to which the remaining portion of the liquid crystal polymer layer is bonded. Thus, with no need to use such release film hitherto required in the practice of the prior art method, not only can the single-sided metal-foil laminates be easily obtained, but also two single-sided metal-foil laminates can be obtained at a time through a single peel-off process and the speed of manufacture of the laminate is accordingly substantially twice that required to manufacture the single laminate.
In the practice of the single-sided metal-polymer laminate making method of the present invention, the double-sided metal-polymer laminate is preferably made by sandwiching the liquid crystal polymer film between two metallic foils to provide a sandwich structure which is subsequently hot-pressed by means of a hot-press. The single-sided metal-polymer laminates according to the present invention are manufactured by means of the method of the present invention described hereinabove.
The single-sided metal-polymer laminate of the present invention includes the liquid crystal polymer layer having a thickness preferably not greater than 15 xcexcm. Also, the liquid crystal polymer layer in the single-sided metal-polymer laminate of the present invention has a segment orientation ratio (SOR) which is preferably not greater than 1.3.
The parts-mounted circuit board of the present invention makes use of the above described single-sided metal-polymer laminate on which electronic component parts are mounted and connected electrically.
The multi-layered parts-mounted circuit board of the present invention is of a structure in which the single-sided metal-polymer laminate of the present invention is laminated with a similar single-sided metal-polymer laminate or any other laminate, and electronic component parts are mounted on at least one surface of the multi-layered parts-mounted circuit board.
In the practice of the method of making the double-sided metal-polymer laminate according to the present invention, a metal foil is applied to and is then hot-pressed to one of opposite surfaces of the liquid crystal polymer layer of a single-sided metal-polymer laminate remote from the metal foil to thereby complete the double-sided metal-polymer laminate. The double-sided metal-foil laminate of the present invention is manufactured in this manner.
The apparatus for making the single-sided metal-polymer laminate according to the present invention comprises a hot-press device for hot-pressing the liquid crystal polymer film sandwiched between first and second metal foils in a layered structure in a direction across the thickness of the liquid crystal polymer film, and a separating device for separating the resultant double-sided metal-polymer laminate into first and second single-sided metal-polymer laminates along a plane intermediate of the thickness of the double-sided metal-polymer laminate.
As hereinbefore discussed, one of the important features of the present invention lies in the utilization of the intra-layer separability owned by the liquid crystal polymer layer, that is, the capability of the liquid crystal polymer layer being separated internally into two polymer plies, so that the intended single-sided metal-polymer laminate can be obtained without allowing the unique intra-layer separability to be lost during the manufacture thereof For this purpose, it is essential that the liquid crystal polymer layer even though softened is not caused to melt and, therefore, the temperature of the liquid crystal polymer layer should not exceed the melting point thereof However, the liquid crystal polymer layer does not always has a fixed melting point, and the melting point thereof may vary depending on the thermal history applied to the liquid crystal polymer layer. By way of example, if the liquid crystal polymer film or layer is placed in the atmosphere of a temperature close to, but lower than the melting point thereof (for example, consistently in the atmosphere of a temperature lower by 15xc2x0 C. than the melting point thereof), the melting point at the starting time will increase with time and will finally increase to a value higher by about 120xc2x0 C. than the melting point thereof at the starting time. Thus, at the time the melting point of the liquid crystal polymer film or layer has increased to the temperature higher than that at the starting time, the intra-separability of the liquid crystal polymer film or layer will not be lost provided that it is heated to a temperature lower than the increased melting point thereof.
In the practice of the present invention, the hot-pressing may be carried out by the use of a hot-press machine, a vacuum hot-press machine, or a hot roll press. Alternatively, a press machine, a vacuum press machine or a roll press, each having a separate heating means installed substantially adjacent thereto, can be used.
The single-sided metal-foil laminate of the present invention can be used not only as a material for circuit substrates, but also in a variety of applications in which laminates of general-purpose plastics with metal foils are employed. However, particularly where the single-sided metal-foil laminate of the present invention is used as a material for circuit substrates, it is desirable that the physical property such as the thermal expansion coefficient of the liquid crystal polymer film in a film forming direction is equal to or substantially equal to that in a direction perpendicular to the film forming direction. Considering, however, that the liquid crystal polymer molecules tend to be easily oriented, formation of the liquid crystal polymer film with the standard film forming method would result in that the liquid crystal polymer forming the film will have its molecules strongly oriented (with SOR not smaller than 1.5) in the film forming direction. Where the liquid crystal polymer film having the molecules strongly oriented in the film forming direction is used as a material for the single-sided metal-polymer laminate, the liquid crystal polymer layer in the resultant single-sided metal-polymer laminate will have its molecules strongly oriented in the film forming direction as is the case with the raw material film and, therefore, the physical property such as the thermal expansion coefficient in the film forming direction will become different from that in the direction perpendicular to the film forming direction.
In view of the foregoing, the single-sided metal-polymer laminate for use as a material for the circuit substrates is preferably of a nature in which the liquid crystal polymer film used therein has an isotropy (with SOR not greater than 1.3, and ideally 1).
As hereinbefore discussed, the present invention is intended to provide the laminate comprising the electric insulating layer made of the liquid crystal polymer and the metallic foil layer, wherein the liquid crystal polymer layer can be made thin, and, in particular, to provide as a suitable material for the circuit substrates the laminate in which the electric insulating layer made of the liquid crystal polymer has an isotropic molecular orientation. Accordingly, the liquid crystal polymer layer used in the single-sided metal-polymer laminate of the present invention makes it possible to provide the circuit substrate comprising the thin liquid crystal polymer layer and the metal foil layer, which substrate has long been desired for, because of the feature of the thin thickness secured and because of the isotropic molecule orientation.